1. Technical Field
The present invention relates to a thermal transfer sheet and an intermediate transfer recording medium that can yield prints having thermally transferred images, which possess excellent various fastness or resistance properties even under severe service conditions, and can realize the transfer of a transfer portion or a protective layer onto an object with good transferability.
2. Prior Art
Thermal transfer has become extensively used as a simple printing method. The thermal transfer is a method which comprises the steps of: putting a thermal transfer sheet, comprising a colorant layer provided on one side of a substrate sheet, on top of a thermal transfer image-receiving sheet optionally provided with an image-receptive layer; and image-wise heating the backside of the thermal transfer sheet by heating means such as a thermal head to selectively transfer the colorant contained in the colorant layer to form an image on the thermal transfer image-receiving sheet.
Thermal transfer methods are classified into thermal ink transfer (hot melt-type thermal transfer) and thermal dye sublimation transfer (sublimation-type thermal transfer). The thermal ink transfer is a method for image formation wherein a thermal transfer sheet comprising a substrate sheet, such as a PET film, bearing thereon a hot-melt ink layer, comprising a dispersion of a colorant, such as a pigment, in a binder, such as a hot-melt wax or resin, is provided and energy according to image information is applied to heating means such as a thermal head to transfer the colorant together with the binder onto a thermal transfer image-receiving sheet such as paper or plastic sheets. Images produced by the thermal ink transfer have high density and possess high sharpness and are suitable for recording binary images of characters or the like.
On the other hand, the thermal dye sublimation transfer is a method for image formation which comprises the steps of: providing a thermal transfer sheet comprising a substrate sheet, such as a PET film, bearing thereon a dye layer comprising a dye, which is mainly thermally transferred by sublimation, dissolved or dispersed in a resin binder; and applying energy according to image information to heating means such as a thermal head to transfer only the dye onto a thermal transfer image-receiving sheet comprising a substrate sheet, such as paper or a plastic, optionally provided with a dye-receptive layer. The thermal dye sublimation transfer can regulate the amount of the dye transferred according to the quantity of energy applied and thus can form gradation images of which the image density has been regulated dot by dot of the thermal head. Further, since the colorant used is a dye, the formed image is transparent, and the reproduction of intermediate colors produced by superimposing different color dyes on top of each other or one another is excellent. Accordingly, high-quality photograph-like full color images can be formed with excellent reproduction of intermediate colors by transferring different color dyes, such as yellow, magenta, cyan, and black, onto a thermal transfer image-receiving sheet, so as to superimpose the color dyes on top of each other or one another, from a thermal transfer sheet of the different colors.
These thermal transfer methods can simply form various images and thus have become utilized in the formation of prints the number of which may be relatively small. Thermal transfer sheets used with these thermal transfer methods have various practical applications. Representative examples of applications include proof sheets, and recording sheets for output images, output plans or designs drawn by CAD/CAM or the like, or images output from a variety of medical analyzers or measuring instruments such as CT scanners and endoscopic cameras. They can also be used as the alternative of instant photographs, and as paper for producing identity certifications, ID cards, credit cards, and other cards on which facial photographs or the like are printed, or for producing synthetic or memorial photographs which are taken at amusement facilities such as recreation parks, game centers, museums, aquariums and the like.
When ID cards such as identity certifications are prepared using the above thermal transfer sheet, the thermal ink transfer method can easily form binary images of characters, numerals and the like. The thermal ink transfer method, however, is unsuitable for the formation of images required to have high quality, such as a photograph of a face. Further, the formed images are disadvantageously poor in fastness or resistance properties, particularly abrasion resistance. On the other hand, the thermal dye sublimation transfer is suitable for the formation of gradation images such as a photograph of a face. Unlike images formed using printing ink, the images formed by thermal dye sublimation transfer, however, do not contain any vehicle and thus are disadvantageously poor in fastness and resistance properties such as lightfastness and weathering resistance.
In order to overcome the above drawbacks, a method has been adopted wherein a transparent film is laminated onto the surface of the formed image. This method, however, involves a complicate operation. In addition, in this method, since lamination is carried out on the whole object, curling occurs in the object. Further, a very thin film cannot be used for reasons of laminating operation. This inevitably increases the thickness of the whole print.
In order to solve these drawbacks, a method has been proposed wherein a protective layer transfer sheet comprising a substrate film and a transferable resin layer (a protective layer) provided on the substrate film is provided and the transferable resin layer is transferred to provide a protective layer on at least a part of the image. According to this method, fastness or resistance properties such as chemical resistance and lightfastness can be improved to some extent. The fastness and resistance properties of the thermally transferred image, however, are not yet satisfactory under severe practical service conditions. The protective layer transfer sheet is advantageous in that the size of the protective layer (resin layer) transferred can be properly varied, but on the other hand, a tendency toward an increase in protective layer transfer processing speed has made it difficult to transfer the protective layer with good transferability. The protective layer transfer sheet has many additional disadvantages including that, when the fastness or resistance properties of the protective layer, such as heat resistance is improved, heat applied at the time of the transfer of the protective layer is likely to deteriorate the transferability.
The diversification of the applications has led to a demand for the formation of a thermally transferred image on a desired object. A method has been proposed, as one method for meeting this demand, wherein a colorant such as a dye or a pigment is transferred, from a thermal transfer sheet comprising a dye layer or a hot-melt ink layer, onto a receptive layer in an intermediate transfer recording medium comprising the receptive layer separably provided on a substrate to form an image on the receptive layer and, thereafter, the intermediate transfer recording medium is heated to transfer the receptive layer, with the image formed thereon, onto an object (Japanese Patent Laid-Open No. 238791/1987 or the like).
Since the use of the intermediate transfer recording medium permits the receptive layer to be transferred onto an object, this method is preferably used, for example, for objects, onto which a colorant is less likely to be transferred making it impossible to form high-quality images directly on them, and objects which are likely to be fused to the colorant layer at the time of thermal transfer. Further, a method may be adopted wherein necessary matter such as a signature is previously written or printed on an object and, thereafter, a transfer portion, with an image of characters, photographs or the like being formed thereon, is transferred from an intermediate transfer recording medium. Therefore, the intermediate transfer recording medium is preferably used in the preparation of passports or other identity certifications, credit cards/ID cards, or other prints.
The present applicant has proposed in Japanese Patent Laid-Open No. 315639/1998 a receptive layer transfer sheet (an intermediate transfer medium film) wherein, in order to impart fastness and resistance properties, such as lightfastness, weathering resistance, and abrasion resistance, to thermally transferred images on objects such as ID cards, an ionizing radiation-cured resin layer and a receptive layer are separably provided on a substrate.
When the above intermediate transfer recording medium is used, the fastness and resistance properties of the thermally transferred image can be improved. However, the hardness of the ionizing radiation-cured resin layer is so hard that the ionizing radiation-cured resin layer cannot conform to the flexibility of the object, leading to the deformation of the object between the transfer portion and the non-transfer portion of the intermediate transfer recording medium. Further, in transferring the intermediate transfer recording medium onto an object, due to a tendency toward an increase in transfer processing speed, the transferability of the intermediate transfer recording medium is unsatisfactory, and, in this case, for example, uneven edge of the transferred portion or uneven transferred portion, which is disadvantageous from the practical point of view, occurs.